In the continued quest for new sources of energy, solar heat plays an important part. The sun as a source of energy, however, is not universally available so that direct utilization of its radiation is limited as to both time and location. Thus, there is a need for storage and transportation whenever it is desired to utilize solar energy at a time and/or in a place where there is no sunlight.
Thermal energy may be stored either directly or indirectly. Direct storage utilizes the thermal capacity of various bodies, such as water, walls, rocks or soil, for example. Indirect storage, involving a conversion of sensible heat into latent heat or other forms of energy, makes use of such physical or chemical phenomena as melting, vaporization, solid/solid transformation or dissolution. Direct-storage systems generally require large volumes of heat-storing materials, with attendant high costs and problems of space; in the absence of effective thermal insulation, which in many instances can be achieved only with difficulty or not at all, the occurrence of significant heat losses restricts this technique to short-term storage. Among the various indirect-storage systems, those based on conversion into chemical energy appear to be the most promising since they obviate the aforestated drawbacks and operate nearly without losses over prolonged periods.